This invention relates to synthetic aperture mapping radars and in particular to a beam steering controller algorithm and implementation means for aligning the elevation beam pattern with an isodop.
Modern airborne multimode radars use electronically scanned phased array antennas whose broadside is coincident with the aircraft velocity vector. With such an antenna, the antenna elevation beam axis, the line about which the elevation pattern lies, is a hyperbola on the ground about the cross track vector.
The synthetic aperture modes of these radars are being required to map large swath widths where depression angle coverage is large (40.degree.-45.degree.) and at pointing angles near the aircraft velocity vector. The design of the digital signal processor for these modes is considerably simplified if it can be tuned to a single isodop at a given scan angle. The processor would then "track" an isodop. An isodop, however, is a hyperbola on the ground about the aircraft velocity vector; it is thus naturally misaligned with the antenna elevation beam axis.
A problem that seriously degrades coverage in the wide scan mode pertains to this misalignment between isodops and the area illuminated by the antenna elevation pattern. This has come to be known as the isodop-isogain misalignment problem. Isodops are, of course, lines of constant doppler frequency on the ground; they are hyperbolas about the aircraft velocity vector as shown in FIG. 1. For a phased array radar whose broadside is coincident with the aircraft velocity vector, the antenna elevation pattern also lies along a hyperbola but this hyperbola lies about the crosstrack axis as in FIG. 1. If the array were a single row of elements, then a cone whose axis was the line array and the apex half-angle was the complement of the scan angle, it would define a surface of constant gain. The intersection of this cone with the earth is a hyperbola about the crosstrack. This hyperbola is then a true isogain since the gain along it is constant. As more rows are added and the array acquires a finite elevation aperture, the elevation pattern no longer covers the entire hyperbola, but it still lies along part of this hyperbola in one quadrant. This is a basic property of any phased array antenna. This hyperbola is still considered an isogain although it is no longer a true isogain. One property of the mainbeam being confined to lie along this hyperbola is as follows: if an azimuth beam is defined as being normal to the elevation beam axis at any point in the mainbeam, then the differential gain between any two hyperbolas in the mainbeam, measured along an azimuth beam axis, is constant.
As is shown in FIG. 1, if the antenna scan angle is chosen so that isogain (elevation beam axis) and isodop are aligned at far range, then they diverge or become misaligned at near ranges. This misalignment is particularly acute at small (&lt;30 degrees) scan angles and steep (&gt;20 degrees) depression angles. This partly explains why the misalignment problem does not occur in previous mapping radars; they operated at wide scan angles and shallow depression angles. This misalignment is a problem since the simplest signal processor one can build is tuned to a single isodop for a given scan angle. That is, all range gates are tuned to a single isodop frequency. The processed resolution cells then lie along the isodop. When the isodop and isogain are misaligned, however, the antenna mainlobe will never illuminate the isodop, implying no map can be made then. By the criterion of adequate map S/N, the isodop and isogain must be aligned within plus or minus half an azimuth beamwidth. Azimuth ambiguity depression requirements, however, dictate a slightly more stringent alignment of plus or minus a quarter of an azimuth beamwidth.
One possible solution to the problem is to do nothing, that is, tune the processor to a single frequency without modifying the beam in any manner. The performance of radars operating in accordance with this approach has been clearly unacceptable since a desirable map cannot be made at far aircraft altitudes above 25,000 feet. Increasing the alignment criterion of the S/N ratio to that of half a beamwidth has not been found to increase coverage substantially. The "do nothing" approach therefore is unacceptable from a performance viewpoint indicating that the processor tuning of the antenna pattern must be modified.
Electronically rolling the antenna aperture has also been proposed as a method of aligning the elevation beam along an isodop. Electronic rolling does not reshape the beam at all; it simply reorients the beam to provide a better beam/isodop alignment than was available without electronic roll. A typical criterion for selecting the optimum electronic roll angle is to make the beam a tangent to the isodop at some point (usually the center) in the desired mapping range. The beam is then perfectly aligned with the isodop at map center, but it deviates from it at any other point. Electronic rolling is easily mechanized in the beam controller by simply scaling the direction cosines by the sin/cos of the roll angle. This function is performed in software in the radar computer. Electronic rolling is an adequate solution to the beam/isodop alignment problem when the mapped range swath is a small fraction (10-20%) of the mapping range and mapping depression angles are small (less than 25.degree.). It is not effective however when mapped range swaths are large compared to the mapping range (50% or greater), and mapping depression angles approach 45.degree.. Furthermore detailed examination of the beam pointing command software, however, revealed that inducing a "false" roll to the elevation pattern requires major revision of the existing software, with the result being somewhat clumsy and not efficient. Further, the STC functions required to smooth the return signal level over the swath are very complicated with much ripple. Thus, this solution results in marginal performance and contains mechanization difficulties that render it unattractive.
Another proposed solution to the problem comprehends retuning the doppler frequency of the radar processor from range gate to range gate. Here no reshaping or reorienting of the beam is performed; instead the processor is tuned to map the isodops illuminated by the elevation beam. To implement this, the processor map reference generator (MRG) must calculate the isodop frequency of each range gate on the isogain. The impact of this on the MRG has been found to be substantial; an increase of approximately 500 chips of mainly very high speed hardware is required. The calculations are made particularly cumbersome by the fact that aircraft motion compensation must be included. The basic concept of calculating the individual doppler frequencies has, however, a basic flaw in that the MRG must know the aircraft altitude above each range gate to properly calculate the frequency. In fact, it only knows the altitude directly below the aircraft. Nonflat terrain can then result in incorrect isodop frequencies calculated for various segments of terrain. Terrain variations of only a few hundred inches could cause significant isodop errors with resultant S/N loss. The digital hardware required to compute the tuning frequencies is, therefore extremely complex functionally and very cumbersome to control (via software) for a maneuvering aircraft.
The beam controller of the invention overcomes the problems stated without being subject to the various drawbacks of other solutions and, in contrast, is functionally very simple and easily controlled.